The water-absorbent resin (Super Absorbent Polymer; SAP) is a water-swelling and water-insoluble polymer gelling agent, and is widely used mainly in disposable use as absorbent articles such as disposable diapers, sanitary napkins and the like; water-retention agent for agriculture and gardening; water-stops for industrial use; and the like. As a raw material of such a water-absorbent resin, many monomers or hydrophilic polymers have been proposed. In particular, a polyacrylic acid (salt)-type water-absorbent resin using acrylic acid and/or a salt thereof (hereafter may be referred to also as “acrylic acid (salt)”) as a monomer, is produced most industrially in view of high water absorbing performance thereof.
The water-absorbent resin is generally produced by finely granulating hydrogel, obtained by polymerization of a monomer in an aqueous solution (a hydrogel-like polymer), during polymerization or after polymerization, as needed, and drying it.
As a drying method for hydrogel, there has been known a method for using a belt-type drying machine (PATENT LITERATURES 1 to 5), a method for performing thin film drying using a drum dryer or the like (PATENT LITERATURE 6), a method for performing azeotropic dehydration in an organic solvent (PATENT LITERATURE 7), a method for drying using a fluidized bed (PATENT LITERATURE 8), a method for performing oscillating fluidized drying (PATENT LITERATURE 9), a method for stirring drying using a rotor (PATENT LITERATURE 10), or the like.
In addition, as drying condition of hydrogel, there has been proposed a method for controlling dew point or temperature to enhance property (for example, reduction of a residual monomer, enhancement of absorbency, reduction of a water-soluble component) (PATENT LITERATURES 11 and 12), a method for performing stirring drying by coarse pulverization in the middle of drying (PATENT LITERATURE 13), or the like.
Further, there may be the case where a undried substance (undried gel) generates in a drying step of hydrogel, and because presence of such a undried substance provides excessive load to a pulverization machine during a pulverization step after the drying step, it has also been known a method for removing said undried substance (PATENT LITERATURES 14 to 16). In addition, there has been known a method for specifying fluidity of hydrogel not to produce the undried substance (PATENT LITERATURE 17), a method for using a leveling apparatus of hydrogel in the drying machine (PATENT LITERATURES 18 and 19), a drying method using a constant supplying apparatus for supplying the specific amount of hydrogel to the drying machine (PATENT LITERATURE 20), a method for using infra-red ray or the like in combination, in addition to hot air in drying (PATENT LITERATURE 21), or the like. In addition to these, a method for adding additives such as a surfactant, inorganic fine particles to hydrogel, to enhance drying efficiency (PATENT LITERATURES 22 to 26) has also been known. In addition, a drying method suitable for hydrogel having low neutralization rate (PATENT LITERATURE 27) has also been proposed. In addition, in crushing hydrogel with a solid content of 50 to 70% by weight, using a screw extruder, technique for drying after crushing by supplying 0.1 to 30 parts by weight of water, relative to 100 parts by weight of said hydrogel, preferably drying using the through-flow belt (PATENT LITERATURE 28) has also been proposed.